1. Field of the Invention
The present invention relates generally to an apparatus for forming multilayer sheets, and more particularly to an apparatus having an extrusion die for forming multilayer sheets for photographic printing paper supports, wherein each thickness of the individual layers composing the multilayer sheet is formed to be uniform in a widthwise direction.
2. Description of Related Art
A coextrusion apparatus is usually used for forming multilayer sheets or films for photographic printing paper supports. A feed block die cannot make each of resin layers composing a multilayer sheet have uniform thickness widthwise, in particular if there are great differences in viscosity, temperature and flow rate between melted resins, which form the resin layers. Even if a multi-manifold die, which forms relatively small unevenness of thickness of each layer, is used, it is difficult to obtain multilayer sheets with an extremely small unevenness of thickness of each layer that is desired for the multilayer sheets for the photographic printing paper supports.
To solve these problems, several techniques have been suggested for adjusting the thickness of each layer by means of a multi-manifold die. For example, Japanese Utility Model Provisional Publication No. 54-107664 discloses a multi-manifold die that adjusts they clearances in the entire passages extending from manifolds to a confluence, where the melted resins join one another through the respective passages, with use of a choke bar mechanism. Japanese Patent Provisional Publication No. 52-60853 discloses a method for manufacturing a coextrusion film. According to this method, at least one choke bar facing the confluence adjusts a confluent angle formed by the passages of the melted resins at the confluence. Japanese Utility Model Provisional Publication No. 7-15321 discloses a multilayer sheet forming apparatus providing a passage sectional area adjusting means, which forms a thin plate and pushes and pulls the thin plate along the thickness of the thin plate at the non-passage side of the thin plate, at a part in at least one of plural passages from the manifolds to the confluence. According to Japanese Patent Publication No. 60-37781, manifolds are straight, and there is provided a passage that becomes wider from the center of the die towards both ends thereof, and a slot that becomes narrower from the center of the die towards both ends thereof.
The-above-mentioned techniques, however, have a problem in that it is extremely difficult to eliminate the unevenness of thickness of each of resin layers composing the multilayer sheet along the width thereof or to obtain the multilayer sheet that has a small unevenness of thickness of each layer.
In particular, the above-mentioned arts are disadvantageous to form the multilayer sheets for the photographic printing paper supports, wherein there are great differences in viscosity, temperature and flow rate between the melted resins, which form the resin layers, and it is required that the unevenness of thickness of each layer along the width thereof is xc2x11 xcexcm or less.
The present invention has been developed under the above-described circumstances, and has as its object the provision of a method for forming a multilayer sheet and an extrusion apparatus therefor that is able to form a multilayer sheet composed of resin layers on the condition that melted resins forming the layers are greatly different from one another in viscosity, temperature and flow rate, and unevenness of thickness of each layer along the width thereof is required to be extremely small.
To achieve the above-mentioned object, the present invention is directed to a method for forming a multilayer sheet by means of a multilayer sheet forming apparatus in which a plurality of melted resins join at a confluence through respective passages, the melted resins are piled in layers at the confluence, and the melted resins are extruded through a slot to form the multilayer sheet, broad parts being formed on upstream side of the confluence of at least one passage including a passage for forming the thinnest layer among the passages, the broad parts having clearances which become larger toward both ends of the passage along the width thereof.
Furthermore, to achieve the above-mentioned object, the present invention is directed to a multilayer sheet forming apparatus which forms a multilayer sheet by extruding melted resins from a slot, wherein: the melted resins are supplied to a plurality of manifolds, the supplied resins are extended at the manifolds along widths of passages connecting the manifolds to a confluence, the extended resins are piled in layers at the confluence, and the piled resins are extruded from the slot; a pair of broad parts are formed on at least one of the passages at an upstream side of the confluence, the at least one of the passages including the passage for forming the thinnest layer among the layers, the pair of broad parts having clearances that become larger from broadening start points to both ends of the at least one of the passages along the width thereof; a relation between a total width (W) of the pair of broad parts and a width (L) of the at least one of the passages is Wxe2x89xa70.1 L; and a relation between a clearance (de) of the broad part at the end of the at least one of the passages and a clearance (dc) at the broadening start point is 2.0xe2x89xa7de/dc greater than 1.0.
According to the present invention, the broad parts are formed on at least one passage including the passage for forming the thinnest layer among the passages for respective resin layers of the multilayer sheet at the upstream side of the confluence. In the broad parts, the clearance becomes larger toward both ends of the passage along the width thereof. Consequently, the thickness of the resin layers including the thinnest layer is uniform along the width thereof.